There are 5 checkpoints , including the clean-up checkpoint, in this lab. Work together with your partner using just one of your accounts. CHANGE WHO IS CONTROLLING THE COMPUTER AFTER EACH CHECKPOINT! If you need help with any exercise, raise your hand.

Copy the directory /home/student/Classes/Cs241/Labs/Lab05 to your account.

Change directories into Lab05 and start BlueJ by entering bluej&.

Using the Debugger

Look for the for loop line. Left-click on the left side of that line (if you have line numbers displayed, click on the line number). This will set a breakpoint where the program will stop running, and let you single-step through the code.

When BlueJ reaches the breakpoint you set, stop and examine the values of ALL variables in the program (i, j and AVOGADROS) by reviewing the variables in the lower-right panel of the Debugger window. Click the Continue button to run all lines within the loop and then see how the variable values change. Once you see the patterns of how each variable changes (or does not change), you can close the debugger window by clicking the Terminate button.

Close the VeryLongLoop class editor window.

Open the TabulateFractions class in the BlueJ editor. The main() method is supposed to print a table of the fractions with a given denominator up to and including 1.0. For example, if the input is: 4, the output from the program is

  Fraction        Decimal
       1/4     0.25000000
       2/4     0.50000000
       3/4     0.75000000
       4/4     1.00000000

Compile the class and run its main() method with an input of 4. The table should appear exactly as above. Now we will set a breakpoint in the program and walk through the code line by line.

CAUTION: BlueJ's debugger can be very tempermental, and it is important to use it in just the right way. Follow these directions carefully. If BlueJ hangs or won't let you terminate a program, it may be necessary to kill off BlueJ and restart it.

1. Open the editor window. Use the mouse to set a breakpoint on line number 36, the first line of code after the if() statement. Do this by left-clicking on the line number in the vertical bar on the left side of the window directly to the left of the double decimal = 0.0; statement. (Note: you don't need to have line numbers displayed in order to set breakpoints.) A stop sign will appear.
2. Run the main() method. Enter an input of 4, again. The program will stop with the double decimal = 0.0; statement highlighted. We know it has not yet run because the variable named decimal doesn't appear in the lower-right debugger window.
3. Click the Step button in the debugger window to execute the highlighted line. After it runs, the decimal variable appears in the window.
4. Press the Step button again and again watch the program execute the loop. Between clicking the Step button, you may examine the current state of all the variables in the program. Pay particular attention to the values the decimal variable holds.
5. You will know the main()method has finished running when all the buttons in the debugger turn grey, as does BlueJ's barberpole. You will still need to close BlueJ's terminal window and the debugger window, by clicking on the X in the upper left corner of each.

Now we will re-run the program, but entering an input value that doesn't work (i.e., causes an infinite loop). Run the main() method and use an input value of 6. Leave the breakpoint set, and use Step so we can look at the results after each line runs. (Note: The Step Into is used when we have more methods than main() to jump into. We'll start seeing them in chapter 6.) After the fifth time through the loop, the program should print out

    5/6      0.83333333

    6/6      1.0000000

What happened instead?

Terminate the run of the program by clicking the Terminate button. (If BlueJ does not want to stop the program, you may have to terminate and restart BlueJ. There are notes at the end of this lab on how to do this.)

As you learned in class (we hope :), it is very dangerous to use = = and != with doubles. Change the while loop condition so that the loop continues until decimal is within 0.0001 of the value 1.0.

Compile your class, and re-run the main() method with an input of 6. Try another test case by running the main() method with an input of 7. Have you fixed the bug? Once you are convinced that the bug is gone, remove the breakpoint by clicking on the stop sign.

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The final class in the Debugger project TabulateFractionsAgain is supposed to print the same table as the previous project. Unfortunately, this one, too contains a bug that causes an infinite loop. Use the debugger to find and remove the bug.

1 Be prepared to answer the following questions:

1. In the VeryLongLoop program, could i's value have been printed after the loop had finished?
2. Explain how each of the values were changing (or not).
3. Explain what the bug was in TabulateFractions and how you used the debugger to remove it.
4. Explain what the bug was in TabulateFractionsAgain and how you used the debugger to remove it.

Close project Debugger.

Loops in Graphics

Open CoolCircles in the BlueJ editor and locate the for loop beginning in line 32. The loop is already working as written, and your job is to change the lines within the loop body to give the circles color.

To construct a Color, you must give three integer values red, green, and blue, which stand for the amounts of red, green, and blue, respectively, to mix and create the color. In the current program, the amounts of red, green, and blue used for each color are always the same. This creates the shades of gray.

Modify the values assigned to red, green, and blue so that the circle you draw will have color. We want you to use certain constraints while assigning the values. On each iteration of the loop:

• red, green, and blue must have different values
• red, green, and blue must have their values assigned in terms of the loop variable radius
• red, green, and blue must always have a value between 0 and 255 inclusive for every iteration of the loop

   int red = 255 - radius;
   int red = (radius*radius)%256;
   int red = (int)(radius/5.8)*5;

2 Show us your revised code, and how it runs. Make sure you have used something different than the examples above in at least 2 of the color settings.

Close project Dazzle.

Input Validation

Open project AnswerMe. It contains a single class, OneToTen. Open it in the editor. This program is supposed to keep asking the user for a number between 1 and 10 (inclusive) and to stop asking when it received valid input. However, when you run it, it takes any integer you decide to enter. Modify the program in these ways:

1. You are required to use a do..while loop to accomplish these tasks.
2. Once the user provides valid input, print the value they entered and the number of tries it took them. e.g., if they enter 7 on the first try, the program should print 7 and 1.
3. Give the user up to three (3) chances to enter a number between 1 and 10.
4. If, after three tries, the user still hasn't entered a valid number, print an error message politely telling them that they are "Not Reading Directions!" and terminate.
5. In addition, you may also use an if inside (and/or outside) the do...while if you think it will help.

3 Show us this program when you are ready.

Close project AnswerMe.

Files are Forever

Open the project StudentFile. Compile and run the main() method of the Student class. It should compile and run without problems. It reads the data for single student from a data file and prints the student's name and two exam scores to the output window.

Open the Student class in the editor. First, look at the filename. The File class constructor requires that we specify the entire (absolute) filename path in order for the system to be able to find the file. As you can see, the current code specifies where the original Lab05 files exist. There is also a copy of the file in your Lab05 directory. Modify the filename to point to your copy.

Hint: Type pwd (the present-working-directory command) in a terminal window to find the name of the directory in which you are currently working.

To view the actual file we are reading from, go to terminal window, cd to the Lab05 directory, and type the command more students.dat. The contents of the file should appear on the screen. Note that there is more than one student in the file, but the program currently reads only the first one. Your job is to modify the program so that it processes the entire file.

Return to the Student class in the BlueJ editor window. Add a while loop below the Scanner declaration so that all data in the file will be read. The while loop should look like:

   while (fileIn.hasNext())
   {
     . . . 
   }

4 Show us your completed program.

Close project StudentFile.

Extras for Experts

1. Above the loop have the user enter the name of the student being sought.
2. Within the loop place an if statement that checks to see if the two names are equal. We cannot compare names using ==, instead we need to use the String class' equals method. E.g. s1.equals(s2) checks to see if String s1 is equal to String s2. Only print the name and scores when the student is found.
3. If the student is not found in the file, print an error message when the end of file is reached.

After the Lab

Stopping a Runaway Program in BlueJ

1. You can stop a runaway program in BlueJ by right-clicking the moving arrow in the lower right area of the main BlueJ window. The only choice is "Reset Machine" - choose it and it will stop moving, indicating that the program has stopped.
2. Another easy way to kill BlueJ is to go to its main window and click the X in the upper right corner. This should kill BlueJ including all its windows. 98.375% of the time this works.
3. The next easiest way to kill BlueJ is to right click on the BlueJ item on the task bar at the bottom of your screen. A popup menu appears that includes an option that says kill App. Choose this option.
4. If none of these work, call over your instructor or lab assistant.